1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for transmitting uplink control channels.
2. Description of the Related Art
Recently, in mobile communication systems, a discussion is being held on a technology of applying Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink, which is a transmission link from a terminal (or a User Equipment (UE)) to a base station (or Node B).
FIG. 1 is a block diagram illustrating an internal structure of a transmitter for implementing SC-FDMA in a frequency domain according to the prior art.
Referring to FIG. 1, a channel encoder 101 performs a specific channel encoding process on an input information bit stream. A block encoder, a convolutional encoder, a turbo encoder or a Low Density Parity Check (LDPC) encoder can be used as the channel encoder 101. A channel interleaver 102 performs specific channel interleaving on the input signal output from the channel encoder 101. Although omitted in FIG. 1, a rate matching block composed of a repeater and a puncturer can be interposed between the channel encoder 101 and the channel interleaver 102.
A modulator 103 performs a modulation process, such as Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Q Quadrature Amplitude Modulation (16QAM), etc., on the input signal output from the channel interleaver 102. The output signal of the modulator 103 is multiplied by a gain corresponding to each physical layer channel according to a predetermined rule by means of a relative gain unit 104. A serial-to-parallel converter 105 converts the serial signal output from the relative gain unit 104 into a parallel signal. The output of the serial-to-parallel converter 105 undergoes Fast Fourier Transform (FFT) by means of an FFT unit 106. A sub-carrier mapper 107 maps the FFT-transformed signal to sub-carriers according to a predetermined rule so that a signal of the terminal should occupy only a particular frequency. An Inverse Fast Fourier Transform (IFFT) unit 108 IFFT-transforms the signal output from the sub-carrier mapper 107, and a parallel-to-serial converter 109 converts the parallel signal output from the IFFT unit 108 into a serial signal. A CP adder 110 adds a Cyclic Prefix (CP) to the signal output from the parallel-to-serial converter 109 according a predetermined rule, and the CP-added signal passes through a baseband filter 111, generating a final baseband signal s(t).
FIG. 2 is a block diagram illustrating an internal structure of a receiver for implementing SC-FDMA in a frequency domain.
Referring to FIG. 2, a received signal r(t) is output through a baseband filter 201. Generally, the baseband filter 201 makes a pair with the baseband filter 111 of FIG. 1. The output of the baseband filter 201 is input to a CP remover 202 where a CP part is removed therefrom according to a predetermined rule. The CP-removed serial signal is input to a serial-to-parallel converter 203 where it is converted into a parallel signal. The output of the serial-to-parallel converter 203 undergoes FFT transform in an FFT unit 204, and then input to a sub-carrier demapper 205. The sub-carrier demapper 205 extracts sub-carriers mapped by the sub-carrier mapper 107 of FIG. 1. The extracted sub-carriers are input to a channel equalizer 206 where they undergo a predetermined channel equalization process. Although the channel equalization process includes various possible methods, a detailed description thereof will be omitted herein since the channel equalization methods are not the gist of the present invention. The output of the channel equalizer 206 passes through an IFFT unit 207, and then input to a parallel-to-serial converter 208 where it is converted into a serial signal. The serial signal is input to a demodulator 209 where it undergoes a predetermined demodulation process corresponding to 16QAM, 8PSK, QPSK, etc. The output of the demodulator 209 is input to a channel deinterleaver 210 where it undergoes a predetermined deinterleaving process. The output of the channel deinterleaver 210 is input to a channel decoder 211 where it undergoes a predetermined channel decoding process, detecting final information.
FIG. 3 is a diagram illustrating a scheme for transmitting control channels in an SC-FDMA based system according to the prior art. In FIG. 3, the horizontal axis represents the time domain, and the vertical axis represents the frequency domain.
One SC-FDMA symbol denoted by reference numeral 301 is a symbol generated according to the operation described in FIG. 1, i.e., one CP-added symbol, and it corresponds to an Orthogonal Frequency Division Multiplexing (OFDM) symbol in the normal OFDM system. The SC-FDMA symbol 301 is also called an SC-FDMA block. Reference numeral 302 represents one subframe. The subframe 302 is a time unit in which one packet of data or control information is transmitted, and it corresponds to 1 ms. The subframe 302 is composed of 14 SC-FDMA symbols 301. One slot denoted by reference numeral 303 is 0.5 ms corresponding to a half of the subframe 302, and it is composed of 7 SC-FDMA symbols 301. One subband denoted by reference numeral 304 is a basic unit for resource allocation in the frequency domain, and it is composed of 12 sub-carriers in this system. In FIG. 3, the system is composed of N subbands.
Reference numerals 305 to 308 show a detailed method in which control information is transmitted. The control information corresponds to information such as Channel Quality Indicator (CQI), Acknowledge (ACK)/Negative ACK (NACK), etc. Reference numerals 305 to 308 show how resources such as frequency, code, time, etc. are used in transmitting the control information. Shown by reference numeral 305 is an occasion where a control channel index (control channel ID) #0 is transmitted using time resources corresponding to 7 SC-FDMA symbols of the first slot, using code resource of a Zadoff-Chue (ZC) sequence offset #0, and using frequency resource corresponding to a subband #0. In addition, shown by reference numeral 308 is an occasion where the control channel index (control channel ID) #0 is transmitted using time resources corresponding to 7 SC-FDMA symbols of the second slot, using code resource of a ZC sequence offset #0, and using frequency resource corresponding to a subband #(N−1).
Changing the subband at the slot boundary as stated above is to obtain frequency diversity. The term ‘ZC sequence offset’ as used herein indicates one code resource. That is, the ZC sequence has a property that there is orthogonality between sequences generated by cyclic-shifting one given ZC sequence by differentiating their offsets, making it possible to generate multiple codes (sequences). Therefore, the expression ‘differentiating an offset value’ may mean ‘using different codes’.
The occasion shown by reference numerals 305 to 308 in FIG. 3 will be described with reference to one more example. In the occasion shown by reference numeral 306, a control channel index (control channel ID) #7 is transmitted using time resources corresponding to 7 SC-FDMA symbols of the first slot, using code resource of a ZC sequence offset #2, and using frequency resource corresponding to a subband #1. In the occasion shown by reference numeral 307, a control channel index (control channel ID) #7 is transmitted using time resources corresponding to 7 SC-FDMA symbols of the second slot, using code resource of a ZC sequence offset #2, and using frequency resource a subband #(N−2).
Control information is transmitted from multiple terminals in the method shown by reference numerals 305 to 308 of FIG. 3. For example, the control information is transmitted over the uplink in such a manner that a terminal #1 transmits its control information through a control channel index #1, and a terminal #2 transmits its control information through a control channel index #2.
It can be appreciated from the foregoing description that different control channels use the resources which are orthogonal with each other. That is, since the different control channels use different time resources, frequency resources and code resources, and there is orthogonality between the codes, there is no interference between the control channels in the ideal environment.
Actually, however, since the orthogonal property is not perfect due to a multi-path phenomenon, a Doppler effect, etc. existing in the wireless channel environment, interference may occur between the control channels. In addition, it is general that the orthogonality is not satisfied between the time and code resources used between different cells, causing occurrence of interference.
Regarding the scheme described in FIG. 3, in the environment where interference occurs between control channels or between neighboring cells, when its load is concentrated upon any one subband, there is an inefficiency problem.
For example, let's assume that in a particular subframe, control channels #0˜#5 are all transmitted, but only the control channel #6 among control channels #6˜#11 is transmitted. In this case, the control channel #6 has no interference from other control channels in terms of inter-code interference, but the control channels #0˜#5 may give interference to each other. Generally, it is not possible to prevent the load concentration on a particular subband, and the same problem may occur even in the second slot.